It seems to be common sense, that hypomobility is an essential cause for overweight and adiposity. As a consequence, physical activity usually is recommended against adiposity. In literature doubts, based on empirical studies, could be found, that the lamentations about an unfit and by this fat youth seems to be rather a sociological phenomenon of an imaginary crisis than an empirically based fact.Especially concerning young people in Germany, physical inactivity is accused to be an important cause of adiposity. Therefore we are interested in the spontaneous intensity of physical activity of pupils during their physical education and in leisure time in relation to their body mass index.Pupils (10-19 y old, 15 forms) were observed during their physical education systematically regarding quantitative and qualitative items. Additionally the intensity of physical and sporting activities in leisure time were investigated by questionnaire. The results show, that the rate of underweight was higher than of adiposity in the researched group. Between the intensity of physical activity in physical education and in leisure related to the deviations of normal weight (BMI) no relevant correlations could be stated.Discussing these results, the hypotheses: Physical inactivity is a central cause of adiposity and physical activity is a very good action against adiposity seems to be too simple. Eating behaviour, genetic disposition, psychological factors etc. obviously are very relevant, too. This must be considered in public recommendations against adiposity as well as the risks of uncontrolled physical activities for joints, bones and muscles especially in case of overweight.